diff --git a/README/ReleaseNotes/v642/index.md b/README/ReleaseNotes/v642/index.md
index 284cce2292028..a7b02dc535212 100644
--- a/README/ReleaseNotes/v642/index.md
+++ b/README/ReleaseNotes/v642/index.md
@@ -88,6 +88,21 @@ Users are strongly encouraged to switch to the vectorized CPU backend if they ar
 
 If the vectorized backend does not work for a given use case, **please report it by opening an issue on the ROOT GitHub repository**.
 
+### Default binning of RooFit variables changed to zero bins
+
+A freshly-constructed `RooRealVar` (or `RooErrorVar`) no longer has a default binning of 100 bins.
+Instead, `RooAbsRealLValue::getBins()` now returns `0` until a binning is explicitly set (e.g. via `RooRealVar::setBins()`).
+This makes it possible to distinguish a variable whose binning was deliberately chosen from one that was left at the default, which avoids
+writing redundant `nbins` fields when serializing workspaces to HS3 JSON.
+
+For the cases that previously relied on the default of 100 bins, that value is now injected by the relevant routine:
+unbinned-dataset plotting (`RooAbsRealLValue::frame()`), `RooAbsPdf::generateBinned()`, and `RooAbsRealLValue::createHistogram()`
+all fall back to `RooAbsRealLValue::DefaultNBins` (100) when the variable has no binning set.
+As a result, plotting, generating binned data and creating histograms from a default-constructed variable behave exactly as before.
+
+Code that reads `getBins()`/`numBins()` of a bare variable and expected the value `100` should either set the binning explicitly,
+or read the bin count from the relevant histogram or plot frame instead.
+
 ## Graphics and GUI
 
 ### Store canvas as HTML file
diff --git a/roofit/hs3/src/RooJSONFactoryWSTool.cxx b/roofit/hs3/src/RooJSONFactoryWSTool.cxx
index 65634d6da2a40..b069097820e48 100644
--- a/roofit/hs3/src/RooJSONFactoryWSTool.cxx
+++ b/roofit/hs3/src/RooJSONFactoryWSTool.cxx
@@ -1076,7 +1076,7 @@ void RooJSONFactoryWSTool::exportVariable(const RooAbsArg *v, JSONNode &node, bo
          var["min"] << rrv->getMin();
          var["max"] << rrv->getMax();
       }
-      if (rrv->getBins() != 100 && storeBins) {
+      if (rrv->getBins() != 0 && storeBins) {
          var["nbins"] << rrv->getBins();
       }
       _domains->readVariable(*rrv);
diff --git a/roofit/roofitcore/inc/RooAbsRealLValue.h b/roofit/roofitcore/inc/RooAbsRealLValue.h
index 849a33509d47d..33b1e066b58ab 100644
--- a/roofit/roofitcore/inc/RooAbsRealLValue.h
+++ b/roofit/roofitcore/inc/RooAbsRealLValue.h
@@ -76,6 +76,12 @@ class RooAbsRealLValue : public RooAbsReal, public RooAbsLValue {
   /// Get number of bins of currently defined range.
   /// \param name Optionally, request number of bins for range with given name.
   virtual Int_t getBins(const char* name=nullptr) const { return getBinning(name).numBins(); }
+
+  /// Historical default number of bins, injected by routines that need a
+  /// concrete bin count when a variable has no binning explicitly set
+  /// (i.e. when getBins() returns 0). Used for unbinned-data plotting,
+  /// generateBinned() and createHistogram().
+  static constexpr int DefaultNBins = 100;
   /// Get minimum of currently defined range.
   /// \param name Optionally, request minimum of range with given name.
   virtual double getMin(const char* name=nullptr) const { return getBinning(name).lowBound(); }
diff --git a/roofit/roofitcore/inc/RooUniformBinning.h b/roofit/roofitcore/inc/RooUniformBinning.h
index b30ff9b78da02..76f77241b9b20 100644
--- a/roofit/roofitcore/inc/RooUniformBinning.h
+++ b/roofit/roofitcore/inc/RooUniformBinning.h
@@ -23,7 +23,7 @@ class RooUniformBinning : public RooAbsBinning {
 public:
 
   RooUniformBinning(const char* name=nullptr) : RooAbsBinning{name} {}
-  RooUniformBinning(double xlo, double xhi, Int_t nBins, const char* name=nullptr) ;
+  RooUniformBinning(double xlo, double xhi, Int_t nBins=0, const char* name=nullptr) ;
   RooUniformBinning(const RooUniformBinning& other, const char* name=nullptr) ;
   RooAbsBinning* clone(const char* name=nullptr) const override { return new RooUniformBinning(*this,name?name:GetName()) ; }
 
@@ -48,10 +48,10 @@ class RooUniformBinning : public RooAbsBinning {
 
 protected:
    mutable std::vector<double> _array; ///<! do not persist
-   double _xlo;
-   double _xhi;
-   Int_t _nbins;
-   double _binw;
+   double _xlo = 0.0;
+   double _xhi = 0.0;
+   Int_t _nbins = 0;
+   double _binw = 0.0;
 
    ClassDefOverride(RooUniformBinning, 1) // Uniform binning specification
 };
diff --git a/roofit/roofitcore/src/RooAbsData.cxx b/roofit/roofitcore/src/RooAbsData.cxx
index 830527a7cf9c5..a03e5875f3b7e 100644
--- a/roofit/roofitcore/src/RooAbsData.cxx
+++ b/roofit/roofitcore/src/RooAbsData.cxx
@@ -2543,7 +2543,9 @@ TH2F *RooAbsData::createHistogram(const RooAbsRealLValue &var1, const RooAbsReal
                                   const char *name) const
 {
    checkInit();
-   return createHistogram(var1, var2, var1.getBins(), var2.getBins(), cuts, name);
+   const int nBins1 = var1.getBins()!=0 ? var1.getBins() : RooAbsRealLValue::DefaultNBins;
+   const int nBins2 = var2.getBins()!=0 ? var2.getBins() : RooAbsRealLValue::DefaultNBins;
+   return createHistogram(var1, var2, nBins1, nBins2, cuts, name);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/roofit/roofitcore/src/RooAbsRealLValue.cxx b/roofit/roofitcore/src/RooAbsRealLValue.cxx
index 588e11ef25a97..dafaaaf953a31 100644
--- a/roofit/roofitcore/src/RooAbsRealLValue.cxx
+++ b/roofit/roofitcore/src/RooAbsRealLValue.cxx
@@ -219,7 +219,7 @@ RooPlot* RooAbsRealLValue::frame(const RooLinkedList& cmdList) const
   RooCmdConfig pc("RooAbsRealLValue::frame(" + std::string(GetName()) + ")");
   pc.defineDouble("min","Range",0,getMin()) ;
   pc.defineDouble("max","Range",1,getMax()) ;
-  pc.defineInt("nbins","Bins",0,getBins()) ;
+  pc.defineInt("nbins","Bins",0, getBins()!=0 ? getBins() : DefaultNBins) ;
   pc.defineString("rangeName","RangeWithName",0,"") ;
   pc.defineString("name","Name",0,"") ;
   pc.defineString("title","Title",0,"") ;
@@ -316,7 +316,7 @@ RooPlot *RooAbsRealLValue::frame(double xlo, double xhi, Int_t nbins) const
 
 RooPlot *RooAbsRealLValue::frame(double xlo, double xhi) const
 {
-  return new RooPlot(*this,xlo,xhi,getBins());
+  return new RooPlot(*this,xlo,xhi, getBins()!=0 ? getBins() : DefaultNBins);
 }
 
 
@@ -366,7 +366,7 @@ RooPlot *RooAbsRealLValue::frame() const
     return nullptr ;
   }
 
-  return new RooPlot(*this,getMin(),getMax(),getBins());
+  return new RooPlot(*this,getMin(),getMax(), getBins()!=0 ? getBins() : DefaultNBins);
 }
 
 
@@ -601,6 +601,10 @@ TH1* RooAbsRealLValue::createHistogram(const char *name, const RooLinkedList& cm
     ownBinning[0] = true ;
   } else {
     binning[0] = &getBinning() ;
+    if (binning[0]->numBins() == 0) {
+      binning[0] = new RooUniformBinning(getMin(), getMax(), DefaultNBins) ;
+      ownBinning[0] = true ;
+    }
   }
 
   if (pc.hasProcessed("YVar")) {
@@ -617,6 +621,10 @@ TH1* RooAbsRealLValue::createHistogram(const char *name, const RooLinkedList& cm
       ownBinning[1] = true ;
     } else {
       binning[1] = &yvar.getBinning() ;
+      if (binning[1]->numBins() == 0) {
+        binning[1] = new RooUniformBinning(yvar.getMin(), yvar.getMax(), DefaultNBins) ;
+        ownBinning[1] = true ;
+      }
     }
   }
 
@@ -634,6 +642,10 @@ TH1* RooAbsRealLValue::createHistogram(const char *name, const RooLinkedList& cm
       ownBinning[2] = true ;
     } else {
       binning[2] = &zvar.getBinning() ;
+      if (binning[2]->numBins() == 0) {
+        binning[2] = new RooUniformBinning(zvar.getMin(), zvar.getMax(), DefaultNBins) ;
+        ownBinning[2] = true ;
+      }
     }
   }
 
@@ -668,7 +680,7 @@ TH1F *RooAbsRealLValue::createHistogram(const char *name, const char *yAxisLabel
   RooArgList list(*this) ;
   double xlo = getMin() ;
   double xhi = getMax() ;
-  Int_t nbins = getBins() ;
+  Int_t nbins = getBins()!=0 ? getBins() : DefaultNBins ;
 
   // coverity[ARRAY_VS_SINGLETON]
   return static_cast<TH1F*>(createHistogram(name, list, yAxisLabel, &xlo, &xhi, &nbins));
@@ -755,8 +767,8 @@ TH2F *RooAbsRealLValue::createHistogram(const char *name, const RooAbsRealLValue
   }
 
   if (!nBins2) {
-    nbins_fit[0] = getBins() ;
-    nbins_fit[1] = yvar.getBins() ;
+    nbins_fit[0] = getBins()!=0 ? getBins() : DefaultNBins ;
+    nbins_fit[1] = yvar.getBins()!=0 ? yvar.getBins() : DefaultNBins ;
     nBins2 = nbins_fit ;
   }
 
@@ -836,9 +848,9 @@ TH3F *RooAbsRealLValue::createHistogram(const char *name, const RooAbsRealLValue
   }
 
   if (!nBins2) {
-    nbins_fit[0] = getBins() ;
-    nbins_fit[1] = yvar.getBins() ;
-    nbins_fit[2] = zvar.getBins() ;
+    nbins_fit[0] = getBins()!=0 ? getBins() : DefaultNBins ;
+    nbins_fit[1] = yvar.getBins()!=0 ? yvar.getBins() : DefaultNBins ;
+    nbins_fit[2] = zvar.getBins()!=0 ? zvar.getBins() : DefaultNBins ;
     nBins2 = nbins_fit ;
   }
 
diff --git a/roofit/roofitcore/src/RooDataHist.cxx b/roofit/roofitcore/src/RooDataHist.cxx
index c54b845d9fd88..5a91943a5f9df 100644
--- a/roofit/roofitcore/src/RooDataHist.cxx
+++ b/roofit/roofitcore/src/RooDataHist.cxx
@@ -798,6 +798,17 @@ void RooDataHist::initialize(const char* binningName, bool fillTree)
       }
     }
 
+    // If the variable has no binning explicitly set (the default for a
+    // freshly-constructed RooRealVar, which reports zero bins), materialize the
+    // historical default binning. _vars holds this dataset's own clones (see
+    // RooAbsData::initializeVars, which addClone's the input variables), so this
+    // does not affect the user's original variable.
+    if (RooRealVar* rrv = dynamic_cast<RooRealVar*>(_vars[i])) {
+      if (rrv->getBins() == 0) {
+        rrv->setBinning(RooUniformBinning(rrv->getMin(), rrv->getMax(), RooAbsRealLValue::DefaultNBins));
+      }
+    }
+
     auto lvarg = dynamic_cast<RooAbsLValue*>(_vars[i]);
     assert(lvarg);
     _lvvars.push_back(lvarg);
@@ -1485,7 +1496,14 @@ double RooDataHist::weightInterpolated(const RooArgSet& bin, int intOrder, bool
     double xval = realX.getVal() ;
     double yval = realY.getVal() ;
 
-    RooAbsBinning const& binningY = realY.getBinning();
+    // Use the internal binning of the y variable, not the binning of the
+    // variable passed in `bin`. The latter may be a different object than the
+    // one owned by this RooDataHist (e.g. the histogram observable clone of a
+    // RooHistPdf), with an unrelated default binning. The bin indexing below
+    // relies on `_idxMult` and `centralIdx`, which are both expressed in terms
+    // of the internal binning, so the y binning must match it too. This mirrors
+    // what calcTreeIndex() and interpolateDim() do for the other dimensions.
+    RooAbsBinning const& binningY = static_cast<RooRealVar const&>(*_vars[varInfo.realVarIdx2]).getBinning();
 
     int ybinC = binningY.binNumber(yval) ;
     int ybinLo = ybinC-intOrder/2 - ((yval<binningY.binCenter(ybinC))?1:0) ;
diff --git a/roofit/roofitcore/src/RooErrorVar.cxx b/roofit/roofitcore/src/RooErrorVar.cxx
index af83a963f229e..d8abbcfe06207 100644
--- a/roofit/roofitcore/src/RooErrorVar.cxx
+++ b/roofit/roofitcore/src/RooErrorVar.cxx
@@ -44,7 +44,7 @@ RooErrorVar::RooErrorVar(const char *name, const char *title, const RooRealVar&
   RooAbsRealLValue(name,title),
   _realVar("realVar","RooRealVar with error",this,(RooAbsReal&)input)
 {
-  _binning = std::make_unique<RooUniformBinning>(-1,1,100) ;
+  _binning = std::make_unique<RooUniformBinning>(-1,1) ;
 }
 
 
diff --git a/roofit/roofitcore/src/RooHist.cxx b/roofit/roofitcore/src/RooHist.cxx
index 661ca5d3e1bdc..9d12d66dc53a8 100644
--- a/roofit/roofitcore/src/RooHist.cxx
+++ b/roofit/roofitcore/src/RooHist.cxx
@@ -28,6 +28,7 @@ a RooPlot.
 #include "RooHist.h"
 
 #include "RooAbsRealLValue.h"
+#include "RooUniformBinning.h"
 #include "RooHistError.h"
 #include "RooCurve.h"
 #include "RooMsgService.h"
@@ -302,13 +303,22 @@ RooHist::RooHist(const RooAbsReal &f, RooAbsRealLValue &x, double xErrorFrac, do
   RooProduct scaledFunc{"scaled_func", "scaled_func", {f, RooFit::RooConst(scaleFactor)}};
   std::unique_ptr<RooAbsFunc> funcPtr{scaledFunc.bindVars(x, normVars, true)};
 
-  // calculate the points to add to our curve
-  int xbins = x.numBins();
+  // calculate the points to add to our curve. A variable with no binning
+  // explicitly set reports zero bins; in that case fall back to the historical
+  // default bin count for plotting, using a local binning so that the user's
+  // variable is not mutated.
+  const RooAbsBinning *xbinning = &x.getBinning();
+  std::unique_ptr<RooUniformBinning> defaultBinning;
+  if (x.numBins() == 0) {
+    defaultBinning = std::make_unique<RooUniformBinning>(x.getMin(), x.getMax(), RooAbsRealLValue::DefaultNBins);
+    xbinning = defaultBinning.get();
+  }
+  int xbins = xbinning->numBins();
   RooArgSet nset;
   if(normVars) nset.add(*normVars);
   for(int i=0; i<xbins; ++i){
-    double xval = x.getBinning().binCenter(i);
-    double xwidth = x.getBinning().binWidth(i);
+    double xval = xbinning->binCenter(i);
+    double xwidth = xbinning->binWidth(i);
     Axis_t xval_ax = xval;
     double yval = (*funcPtr)(&xval);
     double yerr = std::sqrt(yval);
diff --git a/roofit/roofitcore/src/RooRealVar.cxx b/roofit/roofitcore/src/RooRealVar.cxx
index 589691d803595..58458da80fa08 100644
--- a/roofit/roofitcore/src/RooRealVar.cxx
+++ b/roofit/roofitcore/src/RooRealVar.cxx
@@ -104,7 +104,7 @@ RooRealVar::RooRealVar()  :  _error(0), _asymErrLo(0), _asymErrHi(0), _binning(n
 RooRealVar::RooRealVar(const char *name, const char *title,
              double value, const char *unit) :
   RooAbsRealLValue(name, title, unit), _error(-1), _asymErrLo(1), _asymErrHi(-1),
-  _binning(new RooUniformBinning(-1,1,100))
+  _binning(new RooUniformBinning(-1,1))
 {
   _value = value ;
   _fast = true ;
@@ -122,7 +122,7 @@ RooRealVar::RooRealVar(const char *name, const char *title,
              double minValue, double maxValue,
              const char *unit) :
   RooAbsRealLValue(name, title, unit), _error(-1), _asymErrLo(1), _asymErrHi(-1),
-  _binning(new RooUniformBinning(minValue,maxValue,100))
+  _binning(new RooUniformBinning(minValue,maxValue))
 {
   _fast = true ;
 
@@ -157,7 +157,7 @@ RooRealVar::RooRealVar(const char *name, const char *title,
              double value, double minValue, double maxValue,
              const char *unit) :
   RooAbsRealLValue(name, title, unit), _error(-1), _asymErrLo(1), _asymErrHi(-1),
-  _binning(new RooUniformBinning(minValue,maxValue,100))
+  _binning(new RooUniformBinning(minValue,maxValue))
 {
     _fast = true ;
     setRange(minValue,maxValue) ;
@@ -815,7 +815,7 @@ void RooRealVar::writeToStream(ostream &os, bool compact) const
       os << " - +INF) ";
    }
 
-   if (getBins() != 100) {
+   if (getBins() != 0) {
       os << "B(" << getBins() << ") ";
    }
 
@@ -868,7 +868,7 @@ void RooRealVar::printExtras(ostream& os) const
   }
   os << ") " ;
 
-  if (getBins()!=100) {
+  if (getBins()!=0) {
     os << "B(" << getBins() << ") " ;
   }
 
diff --git a/roofit/roofitcore/src/RooUniformBinning.cxx b/roofit/roofitcore/src/RooUniformBinning.cxx
index af874a4cb48ef..6829f67678b04 100644
--- a/roofit/roofitcore/src/RooUniformBinning.cxx
+++ b/roofit/roofitcore/src/RooUniformBinning.cxx
@@ -69,7 +69,7 @@ void RooUniformBinning::setRange(double xlo, double xhi)
 
   _xlo = xlo ;
   _xhi = xhi ;
-  _binw = (xhi-xlo)/_nbins ;
+  _binw = _nbins>0 ? (xhi-xlo)/_nbins : 0.0 ;
 
   // Delete any out-of-date boundary arrays at this point
   _array.clear();
diff --git a/roofit/roofitcore/test/stressRooFit_tests.h b/roofit/roofitcore/test/stressRooFit_tests.h
index a29918b92c2f5..6f7ab4ca0a854 100644
--- a/roofit/roofitcore/test/stressRooFit_tests.h
+++ b/roofit/roofitcore/test/stressRooFit_tests.h
@@ -1722,6 +1722,8 @@ class TestBasic306 : public RooUnitTest {
 
       // Plot decay_gm(dt|dterr) at various values of dterr
       RooPlot *frame = dt.frame(Title("Slices of decay(dt|dterr) at various dterr"));
+      // Sample dterr in 100 bins to select the slice values below with setBin()
+      dterr.setBins(100);
       for (int ibin = 0; ibin < 100; ibin += 20) {
          dterr.setBin(ibin);
          decay_gm.plotOn(frame, Normalization(5.), Name(Form("curve_slice_%d", ibin)));
diff --git a/roofit/roofitcore/test/testRooSimultaneous.cxx b/roofit/roofitcore/test/testRooSimultaneous.cxx
index a9b2f34ccb880..31948fb7d2657 100644
--- a/roofit/roofitcore/test/testRooSimultaneous.cxx
+++ b/roofit/roofitcore/test/testRooSimultaneous.cxx
@@ -682,7 +682,10 @@ TEST(RooSimultaneous, PlotProjWDataExtended)
    auto integrateLastCurve = [](RooPlot *plot) {
       const double xmin = plot->getPlotVar()->getMin();
       const double xmax = plot->getPlotVar()->getMax();
-      return plot->getCurve()->average(xmin, xmax) * plot->getPlotVar()->numBins();
+      // Multiply by the number of bins of the plot frame (not the plot
+      // variable, which no longer carries a default binning) to turn the
+      // average events-per-bin of the curve back into a total event count.
+      return plot->getCurve()->average(xmin, xmax) * plot->GetNbinsX();
    };
 
    constexpr double tol = 0.01; // tolerate 1 % sampling error
diff --git a/roofit/roostats/src/AsymptoticCalculator.cxx b/roofit/roostats/src/AsymptoticCalculator.cxx
index 899c604faeb1d..12c9f8a611589 100644
--- a/roofit/roostats/src/AsymptoticCalculator.cxx
+++ b/roofit/roostats/src/AsymptoticCalculator.cxx
@@ -1111,6 +1111,20 @@ RooAbsData * GenerateAsimovDataSinglePdf(const RooAbsPdf & pdf, const RooArgSet
 
     RooArgList obsList(*obs);
 
+    // The Asimov data set is built by looping over the bins of the
+    // observables. A freshly-constructed RooRealVar now defaults to zero bins,
+    // so temporarily materialize the historical default binning for any
+    // observable that has no explicit binning set. The change is undone right
+    // after filling the bins so that the model observables are left untouched.
+    std::vector<RooRealVar *> obsWithDefaultBinning;
+    for (auto *arg : obsList) {
+       auto *rrv = dynamic_cast<RooRealVar *>(arg);
+       if (rrv && rrv->getBins() == 0) {
+          rrv->setBins(RooRealVar::DefaultNBins);
+          obsWithDefaultBinning.push_back(rrv);
+       }
+    }
+
     // loop on observables and on the bins
     if (printLevel >= 2) {
        oocoutI(nullptr,Generation) << "Generating Asimov data for pdf " << pdf.GetName() << std::endl;
@@ -1122,6 +1136,11 @@ RooAbsData * GenerateAsimovDataSinglePdf(const RooAbsPdf & pdf, const RooArgSet
     double binVolume = 1;
     int nbins = 0;
     FillBins(pdf, obsList, *asimovData, obsIndex, binVolume, nbins);
+
+    // restore the zero-bins default on observables that had no explicit binning
+    for (auto *rrv : obsWithDefaultBinning) {
+       rrv->setBins(0);
+    }
     if (printLevel >= 2)
        oocoutI(nullptr,Generation) << "filled from " << pdf.GetName() << "   " << nbins << " nbins " << " volume is " << binVolume << std::endl;
 
diff --git a/tutorials/roofit/roofit/rf306_condpereventerrors.C b/tutorials/roofit/roofit/rf306_condpereventerrors.C
index 091eacb267f41..d068e34f8aa10 100644
--- a/tutorials/roofit/roofit/rf306_condpereventerrors.C
+++ b/tutorials/roofit/roofit/rf306_condpereventerrors.C
@@ -68,6 +68,8 @@ void rf306_condpereventerrors()
 
    // Plot decay_gm(dt|dterr) at various values of dterr
    RooPlot *frame = dt.frame(Title("Slices of decay(dt|dterr) at various dterr"));
+   // Sample dterr in 100 bins to select the slice values below with setBin()
+   dterr.setBins(100);
    for (Int_t ibin = 0; ibin < 100; ibin += 20) {
       dterr.setBin(ibin);
       decay_gm.plotOn(frame, Normalization(5.));
diff --git a/tutorials/roofit/roofit/rf306_condpereventerrors.py b/tutorials/roofit/roofit/rf306_condpereventerrors.py
index aac4ab0a9dd74..3139d63a650a2 100644
--- a/tutorials/roofit/roofit/rf306_condpereventerrors.py
+++ b/tutorials/roofit/roofit/rf306_condpereventerrors.py
@@ -58,6 +58,8 @@
 
 # Plot decay_gm(dt|dterr) at various values of dterr
 frame = dt.frame(Title="Slices of decay(dt|dterr) at various dterr")
+# Sample dterr in 100 bins to select the slice values below with setBin()
+dterr.setBins(100)
 for ibin in range(0, 100, 20):
     dterr.setBin(ibin)
     decay_gm.plotOn(frame, Normalization=5.0)